The use of mechanical coupling sleeves to join concrete reinforcing bars together in end-to-end relation is well known. A constant problem in connection with such mechanical couplers is the fact that the reinforcing bars are ribbed or formed with protrusions that can interfere with the coupler operation.
One approach to the mechanical coupling of reinforcing bars is shown in my U.S. Pat. No. 3,253,332 in which the bar ends are gripped by convergently acting wedges. This approach requires that the ribs on the bar ends be removed to enable the wedges to securely grip the bar ends. A second approach to mechanical coupling is set forth in U.S. Pat. No. 3,415,552 in which a threaded tapered connecting section is formed on the end of the reinforcing bar, including the ribs of the bar. The formation of a tapered thread on the end of a reinforcing bar can be accomplished by means of a number of conventional machine tools, but can probably most easily be accomplished by means of a thread mill and a multiple ring cutter.
Typically, reinforcing bars ranging from about 1 to about 21/2 inches in diameter and having a length of about 20 to 30 feet, or more, will be threaded with a tapered thread at one or both ends by means of a thread mill. It has been found, however, that unless the thread mill is extremely large, problems sometimes occur in connection with gripping the ribbed reinforcing bars. These problems are complicated by the fact that the ribbed reinforcing bar is often made to rather crude specifications and may, for example, be irregularly formed or deformed in the area which must be gripped during the threading process. Thus, the combination of bar irregularity and ribs makes it difficult to grip concrete reinforcing bars with enough rigidity to ensure the formation of accurate tapered threads. Thus, the conventional chuck assemblies, such as collet-type or pivotal jaws are inadequate for the job or rigidly gripping a concrete reinforcing bar, with its ribs or protrusions and inherent inaccuracies of manufacture, unless the chuck assembly is massively oversized.
The thread mill threading process which is preferred for the formation of tapered threads on reinforcing bars is described in more detail in my Canadian Pat. No. 1,034,307, and a union for the tapered threaded reinforcing bar is set forth in my U.S. Pat. No. 3,850,535.